Lecture 23: Trophic Relationships and Plant-Animal Interactions

How do we organise feeding communities in an ecosystem?

Using trophic levels.

What are the categories of trophic levels?

Define primary producers.

Organisms who make their own food.

Define primary consumers.

Organisms that eat plants (herbivores).

Define secondary consumers.

Organisms that eat herbivores (carnivores).

Define tertiary consumers.

Organisms that eat secondary consumers (also carnivores).

Define decomposers.

Organisms who eat dead organic matter.

Why are trophic levels organised into a pyramid?

This is because we expect there to be more biomass in lower trophic levels compared to higher trophic levels.

We expect this because energy transfer from a lower trophic level to a higher one is inefficient and so a lot of energy is lost. Therefore, after a certain point, there isn’t enough energy to support another trophic level.

Also, since animals tend to be larger in higher trophic levels, the amount of energy one organism needs in a higher trophic level is much more than what one organism in a lower trophic level needs. Hence, with an already lower amount of energy available, the biomass at higher trophic levels is lower.

Why are trophic levels not infinite?

Because eventually there isn’t enough energy from the biomass in the trophic level to sustain another trophic level.

Define food chain.

A linear sequence of organisms that shows the energy transfer from one organism to another as they eat each other.

Define food web.

A visual representation that shows many interconnected food chains. This to represent all the feeding relationships in a community.

Define parisitoid.

An insect that lays eggs inside the body of another insect. The insect then grows in that insect, parasitising the host.

What is an example of interactions in a ecological community? Specifically, parasitoid.

1. Parasitoid insects lay eggs into caterpillars (herbivores)

2. Caterpillars then eat plants (primary consumers)

*this shows the interactions between species

What explains the fact that there are so many different types of interactions between species?

Indirect effects.

Define indirect effects.

When species A affects the influence species B has on species C. Therefore, species A is indirectly affecting species C.

How is scramble/exploitative competition an example of an indirect effect?

Scramble/exploitative competition is an example ONLY IF the resource being competed for is another species (predation/herbivory).

• In the picture, species 1 has a direct effect on species 2 since its eating it (via predation)

• species 3 also has a direct effect on species 2 because its eating it (via predation)

• however, species 1 and 3 have an indirect negative effect on each other because they’re competing for the same resource.

• Hence, species 1 affects the influence of species 3 on species 2 (and vice versa) because if species 3 is not available, species 2 has a positive indirect effect on its population because it’s able to survive. Also, if species 1 is not available then species 2 also has a positive indirect effect on its population.

What is another example of an indirect effect?

Trophic cascade.

Define trophic cascade.

When interactions between animals on 2 different trophic levels affect a third trophic level.

Explain the process of a trophic cascade.

• secondary consumers (predators - carnivores) have a direct effect on primary consumers (herbivores)

• herbivores have a direct effect on primary producers (plants)

• however, the interaction between the secondary consumer and primary consumer can cause the secondary consumer to have a direct positive effect on the primary producers

• this happens because secondary consumers can keep the population of primary consumers low. Thus, letting less herbivores eat the plants.

What is the hypothesis about trophic cascades?

Hairston, Smith and Slobodkin (HSS - 1960), came up with the Green World Theory.

They created this hypothesis to explain why the earth looks green when you see a picture of it.

They wanted to know why herbivores don’t just eat it all.

However, they proposed that the world is full of green because carnivores keep the herbivore population low so herbivores can’t eat all the plants.

*this is an example of an indirect effect.

What is a pattern on the effects between different trophic levels in a trophic cascades?

Tends to have alternating effects across trophic levels.

Define top-down control.

When the population size of a lower trophic level is maintained by an organism from a higher trophic level.

ie. secondary consumers (predators) keeping primary consumer populations (herbivores) low

How do you test top-down control?

Remove the predator from the environment and see if the organism one trophic level down increases in population size.

Define bottom-up control.

When the population of an organism is kept low because there is not enough food. Thus, it’s a lower trophic level that is maintaining the population size of a higher trophic level.

How do you test for bottom-up control.

Increase the amount of resources in the ecosystem and see if the population size of the species increases or not.

What is an example of indirect effects and trophic cascades in real life?

Spiller and Schoener experiment in the Caribbean.

Food chain identified there:

• beetles eat the plants

• spiders and anolis lizards eat the beetles

• anolis lizards also eat the web-spinning spiders

They found that the anolis lizards have a positive direct affect on plants, that they limit the population growth of beetles because they eat a lot of them, and the effect of lizards on spiders is weak.

*therefore, plants grew better with lizards compared to without.

How come you can’t always tell how the trophic cascade will go?

Organisms don’t just eat one species and so there can be positive or negative indirect effects on populations depending on which species is more affected by the predation/herbivory.

In the HSS experiment, the anolis lizards ate both the beetles and spiders. However, the lizards could be good or bad on the plants depending on how they interacted with the beetles or spiders.

• For example, if the anolis lizards really drive down the spider population, less beetles will be eaten and the indirect effect on plants will be negative (since there are more beetles around to eat them). *works for beetles too

• However, if anolis lizards have a weak effect on the spider population, then the population of beetles will decrease and less plants will be eaten. Therefore, creating an indirect positive effect on the plants. *works for beetles too

How can you tell how a trophic cascade will really go?

Experiments must be done because there are so many different factors that can affect how things will go.

*think about how the HSS experiment had to be tested

How are direct and indirect affects shown on a model?

solid line = direct affect

dashed lines = indirect affect

What did the researcher, Knight, find out about the relationship between larval and adult dragonflies and fish?

• When fish were present, the overall amount of larval dragonflies decreased (not surprising as the fish eat the larval dragonflies). They seemed to have the biggest impact on medium size larvae.

• When fish were present, the overall amount of adult dragonflies decreased as well because the fish had an indirect affect on them (fish eat the baby version of them - thus the adult population must be low too).

What did the researcher, Knight, find out about the relationship between pollination and fish abundance?

If a pond has fish, there are less dragonflies. Hence, pollination increases because more there are less dragonflies to eat the butterflies and bees who pollinate the plants. Notice that there are significantly more bees when ponds have fish.

Therefore, fish has an indirectly positive affect towards the bees AND plants.

Can indirect effects be just as strong, or stronger, than direct effects?

Yes. I mean, look at how many more bees were present due to the indirectly positive effect on fish in the pond.

Define primates.

A type of mammal (ie. humans, apes, lemurs, monkeys).

What did J.B.S Haldane say about beetles?

J.B.S Haldane said that the Creator must love beetles.

Why are most of the species we know insects and plants?

It’s about diet.

It’s very easy for carnivores to eat animal tissues. But it’s hard for them to digest plant tissues. This is because:

• plants have cellulose and lignin which are hard to digest (animals can’t digest them without the help of microbial symbionts - who help break down the fibres).

• plants have a lot of defences against herbivory so that they can stay alive

• there is an arms race between plants and herbivores (specifically insects)

Explain how an arms race between plants and insects (herbivores) is why most of the species we know are plants and insects.

Plants are always trying to evolve new defences while insects (herbivores) are always trying to overcome those new defences.

Hence, there’s this constant change occurring within the plants and insects as a result of a co-evolutionary ‘arms race’ between the two.

This constant evolvement is probably why there is so much biodiversity in plants and insects, and why certain interactions are so specialised.

Why are plants so much more well defenced than animal prey?

Since plants are sessile, they can’t escape by running away (like animals). So they must create defences that are actually useful without the luxury of being able to pick up and go.

Hence, they have evolved really good physical/chemical defences.

Look at this microbial symbiont graph, what does it tell you?

• each dot represents a different species

• the green dots represent herbivores and, in the image, you can see that the microbes present are all quite similar to each other. This is because the species need microbial symbionts to break down the cellulose and lignin in plants

• whereas, the red dots are all carnivores who eat animal tissues and, clearly, they don’t do not have very similar microbial symbionts.

What is an example of a chemical defence in plants?

Milkweeds.

They release a white sap that is poisonous to insects, except Monarch butterfly larva who are specialists to the plants and can evade the defences.

The monarch butterfly larva actually take in the poison form the Milkweeds and store it in their tissues. Then, they use it as their own defence mechanism. Hence, the bright colour warns their predators and tell them “don’t eat me, I’m poisonous.”

We eat plants, so why are we not poisoned?

We artificially select for low poison levels in the plants we eat.

What are the dangers of Cardenolides from Milkweeds?

If humans eat it, we’ll go into cardiac arrest.

Define primary chemicals.

Chemicals that affect our primary metabolism (ie. respiration).

What are some common alkaloids that we as humans use that were originally secondary chemical defences against herbivory?

Hence, we have those compounds because of co-evolution (arms race) between plants and insects.

Why are plant defences so specialised to insects and not vertebrates?

1. Because many insects grow on plants and so they have to find ways to overcome plant defences to just grow.

2. Because vertebrate grazers eat a variety of plants and so they don’t eat high doses of one poison from the plants. Hence, they don’t get the chance to really build a tolerance, unlike insects.

3. Vertebrates have special fermenting chambers in their guts where detoxification by microbes takes place.

Is the physical environment enough to give rise to such a vast amount of biodiversty?

No, because interactions between other organisms is what can produce unlimited diversification (via the co-evolutionary arms race).